One known wind turbine is described in U.S. Pat. No. 5,083,039.
The wind turbine supplies energy to the grid at a given or designated voltage, which must be the same as the grid voltage, and supplies a given or designated current. The grid voltage is determined by the power server to which the wind turbine is connected, whereas current supply depends on wind conditions and server power demand.
The grid voltage is not perfectly constant, but has a reference value about which the grid voltage can vary by roughly 10%; and wind turbine voltage must equal and follow variations in grid voltage.
To maximize conversion of kinetic wind energy to electric energy, modern wind turbines can adapt the speed of the rotor to wind strength, so the voltage and/or current of the electric machine vary in amplitude and frequency, depending on the speed of the rotor.
For the wind turbine to function properly, adjustments are therefore needed, and which are made by the first and second switch converter.
The first switch converter comprises switches, and acts on the electric machine current and/or voltage to control the electric machine and electric energy flow from the electric machine to the DC link circuit.
The second switch converter also comprises switches, and is configured to connect the DC link circuit and the grid, and to control electric energy transfer from the DC link circuit to the grid. More specifically, the second converter acts on respective switches to couple the direct voltage of the DC link circuit to the grid voltage, or vice versa.
The direct voltage of the DC link circuit is fixed, and is set at the design stage to a value of √2 times whichever is higher: the maximum possible voltage of the electric machine, or the maximum possible grid voltage.
So designed, the electric machine can function over a wide range of wind speeds, and the direct voltage always being higher than the electric machine and grid voltages prevents undesired turn-on of the diodes connected to the converter switches. Known turbines pose problems caused by inevitable switching losses, which normally depend on the voltage and current of the switch involved and the time taken for the turbine to switch. Since these values are normally fairly high, the amount of power dissipated by switching on known turbines is significant and has a noticeable effect on performance.
Moreover, when using certain control techniques, such as discontinuous pulse-width modulation, power dissipation also depends on the total number or quantity of switching operations, which varies. Discontinuous pulse-width modulation, in fact, acts on the number or quantity of switching operations per period and the duration of the switching operations per period to adjust the output voltage of the converter, so the amount of power dissipated increases in direct proportion to the number or quantity of switching operations per period.